38 national geographic • February 2016
like the mangrove roots that it swims among. It
took them another four years to discover what
the upper lensed eyes do. The first big clue was
a free-floating weight at the bottom of the rho-
palium that ensures that the upper eye is always
looking upward, even if the jellyfish swims up-
side down. If this eye detects dark patches, the
jellyfish senses that it’s swimming beneath the
mangrove canopy, where it can find the small
crustaceans that it eats. If it sees only bright
light, it has strayed into open water, and risks
starving. With the help of its eyes, this brainless
blob can find food, avoid obstacles, and survive.
The box jellyfish’s eyes are part of an almost
endless variation of eyes in the animal king-
dom. Some see only in black and white; others
perceive the full rainbow and beyond, to forms
of light invisible to our eyes. Some can’t even
gauge the direction of incoming light; others
can spot running prey miles away. The small-
est animal eyes, adorning the heads of fairy
wasps, are barely bigger than an amoeba; the
biggest are the size of dinner plates, and belong
to gigantic squid species. The squid’s eye, like
ours, works as a camera does, with a single lens
focusing light onto a single retina, full of photo-
receptors—cells that absorb photons and con-
vert their energy into an electrical signal. By
But that’s not true. It’s not true at all.”
In his lab at Lund University in Sweden,
Dan-Eric Nilsson is contemplating the eyes of a
box jellyfish. Nilsson’s eyes, of which he has two,
are ice blue and forward facing. In contrast, the
box jelly boasts 24 eyes, which are dark brown
and grouped into four clusters called rhopalia.
Nilsson shows me a model of one in his office: It
looks like a golf ball that has sprouted tumors. A
flexible stalk anchors it to the jellyfish.
“ When I first saw them, I didn’t believe my
own eyes,” says Nilsson. “They just look weird.”
Four of the six eyes in each rhopalium are
simple light-detecting slits and pits. But the
other two are surprisingly sophisticated; like
Nilsson’s eyes, they have light-focusing lenses
and can see images, albeit at lower resolution.
Nilsson uses his eyes to, among other things,
gather information about the diversity of animal
vision. But what about the box jelly? It is among
the simplest of animals, just a gelatinous, pulsat-
ing blob with four trailing bundles of stinging
tentacles. It doesn’t even have a proper brain—
merely a ring of neurons running around its bell.
What information could it possibly need?
In 2007, Nilsson and his team demonstrated
that the box jelly Tripedalia cystophora uses its
lower lensed eyes to spot approaching obstacles,
‘IF YOU ASK PEOPLE WHAT ANIMAL
EYES ARE USED FOR, THEY’LL SAY:
SAME THING AS HUMAN EYES.
By Ed Yong
Photographs by David Liittschwager